For many years I've used a basic Kensington Orbital. I've got one on my desk at work. Our computers get refreshed every three years, but the IT guys know anyone who tries to take away my trackball will lose a hand.
I love the Kensington Orbital, flicking the trackwheel has become second nature for scrolling through console output or long webpages. I do miss a middle button occasionally though.
I might take a look at one of these Elecom trackballs or the 'expert' Kensington with four buttons
